1. Field of the Invention
The present invention relates to a technique in which a steel product is heat-treated by using an induction heating device.
2. Description of the Related Art
In the process of steels, various heat treatments, such as quenching, tempering, annealing, or the like, are carried out in order to manufacture an even stronger and tougher steel product due to the characteristics, such as hardness, toughness, and the like, of the steel product which will be a product being improved. These heat treatments are generally divided into a process of heating and a process of cooling. In the heating process thereamong, a transformation temperature corresponding to a component of a steel product is a standard. For example, heating must be carried out so that a temperature of a steel product is higher than the transformation temperature in the case of quenching, and heating must be carried out so that a temperature of the steel product is lower than the transformation temperature in the cases of tempering and annealing.
Therefore, it is necessary to high-accurately heat in accordance with a purpose of a heat treatment. Further, in order to suppress dispersion in the quality in the same member, it is necessary to uniformly heat over the inside and outside of the steel product. This heat treatment method is called a uniform heating.
Further, because a steel product which is generally manufactured and to which the heat treatments of quenching and tempering are applied, is cooled from, mainly, the surface thereof, the hardness in the surface tends to be higher than that of the inside. Such a steel product having a hardness distribution in the thickness direction is irresistible for a corrosive environment, and it has been clear that, if such a steel product is used in the ocean, or for a pipeline of petroleum or natural gas, and the like, a stress corrosion cracking easily arises.
Accordingly, there are cases of carrying out processing in which a hardness difference between the surface layer portion and the inside is made small due to the surface layer portion being heated at a high temperature. This heat treatment method is called a surface layer heating.
Conventionally, as a heating method for achieving these heating conditions, for example, a heat treatment technique which is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 9-170021, and which uses an induction heating device, is known.
In the disclosed art, an induction heating method has been proposed in which a semi-heating step of induction-heating at the same frequency as in the induction heating at a heating step and with inputted electric power being made lower than that at the heating step, is provided between the heating step in which a temperature of the steel product is risen in an induction heater, and a soaking step of heating with a frequency being made higher than that of the heating step and with inputted electric power being made lower.
However, it is not efficient because several ten minutes are required as a heating time in the art disclosed in JP-A No. 9-170021. Further, because a frequency of the induction heating device is changed on the way of heating of a steel product, it is necessary to equip a mechanism of switching a frequency. Accordingly, the apparatus is made high-priced, and the structure of the apparatus is further made complex. Further, in an inputted electric power calculation for heating a steel product, an induced current distribution at the inside of a steel product, releasing of heat by the atmosphere, an efficiency of a heating device, specific heat of a steel product, and the like which are the factors required for achieving an high-accurate temperature control, are not considered.